Diesels Do Not Have PCV Valves!

[TDI Believer]

Just thought I'd mention that the crankcase ventillation systems employed on diesels are not PCV (Positive Crankcase Ventillation) systems. They are Closed Crankcase Ventillation (CCV) systems.

A PCV system utilizes strong engine vacuum to draw gases and vapors out of the crankcase via a one-way check valve known as the PCV valve. These systems also generally include a breather cap/filter or tube to let air into the crankcase so that the crankcase is neither pressurized or depressurized.

Diesels do not produce sufficient vacuum for a PCV system. They use only a crankcase breather which is vented to the filtered side of the intake ducting. The engine can "breathe" both in and out via the breather cap. The round black thing on the TDI valve cover is a breather cap.

On both PCV and CCV systems, the breather cap usually employs a baffle or mesh for the purpose of preventing oil from splashing into the vent tube and entering the intake.

All this talk of PCV valves on the TDI was starting to get to me.

Rich

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Rich in Northern VA
2000 Jetta GLS TDI Manual
Canyon Red/Beige
w/Lux/Cold/6CD/Mikro-Audio/BORA Badge
VA Tag "BORA TDI"

[This message has been edited by TDI Believer (edited May 31, 2000).]

 

[GeWilli]

So that is why the Service rep said flat out that the TDI doesn't have a PCV? But it does have a CV just that instead of being a P it is a C. Hmmm

Well, maybe that is why that Racor unit is called a CCV? Since it is designed for Diesels. . . .

Thanks for the correction

 

[TDI Believer]

Right! The ventillation on our diesels is not "positive", that is, there is no positive/continuous flow of air/gases/vapors through a breather, into the crankcase, and drawn out though a PCV valve via vacuum. The ventillation on a diesel is passive, the crankcase simply breathes through the cap.

 

[Karl Roenick]

Under what condition would it breath in?

 

[TDI Believer]

Probably not much. The idea is to keep the pressure inside the crankcase pretty much equal with the pressure outside so as not to cause gasket failure via a pressure differential. Given good rings and high-quality oil, it probably doesn't breathe out that much either. The system is closed (not vented to atmosphere) to recover and burn anything that is breathed out.

 

[jmclain]

Hmmm...interesting. Good info, thanks.

 

[Karl Roenick]

This makes sense, but I don't think my 94 gas VW had that either, but I could be wrong.
I just looked at the Bentley manual. It refers to a Positive Crancase Ventilation valve. I didn't believe that anything valve like happens with this component.
It could be that Bentley just screwed up again, or that there is a requirement to call certain things by an agreed upon name, or jargon (?).

 

[TDI Believer]

Bentley likely screwed up.

Your '94 gasser had to have either:

1) PCV system
2) CCV system (I've seen this on gassers)
3) Vent to atmosphere

If the crankcase is not vented, the engine will eventually blow a gasket or seal due to pressurization of the crankcase from blow-by gases.

Keep in mind a "valve" is used to either partially or completely restrict flow. A check valve restricts flow to one direction only. PCV valves are check valves. If you blow into the top of one it should close. If you suck, it should open.

[This message has been edited by TDI Believer (edited May 31, 2000).]

 

[Craig]

There may not be much vacuum on a diesel, but there is SOME in the post air filter track, or valois' propane system would not be working. If the vent would breath in as much as it breaths out you would not find so much oil in the tubing between the valve and the intake after a short time. The question is how much does it breath in, if any? With the PVC bypass system, unfiltered air would be drawn in. I don't believe that there is ever any negative pressure in the crank. There is certainly extremely high positive pressure as I know only too well from experience.

 

[mickey]

Whatever.

-mickey

 

[TDI Believer]

Like I said, it probably doesn't breathe in much at all. Times when it might breathe in include:

1) Change in atmospheric pressure with the engine off

2) When draining your oil during an oil change

While I wouldn't worry much about dirt getting into the engine that way, it is still a good idea to put a filter on the other end of the elephant hose if you've installed one.

 

[Ric Woodruff]

What about blowby? That creates a positive constant pressure in the crankcase that needs to be vented. Even a just-broken-in engine still has SOME blowby.

------------------
Ric Woodruff

Braumeister von Sehr Gutem Bier
Since the Last Millennium

1998 Jetta TDI Sport

Great band at: www.carolinespine.com

 

[Ric Woodruff]

What if it is stuck in the open position?

 

[TDI Believer]

Ric,

On our TDIs, blow-by gases flow through the breather cap because of excess pressure in the crankcase. The minor vacuum of the TDI intake will assist this to a minor degree.

In a PCV system on a gasser, the gases are sucked out via manifold vacuum applied to the PCV valve. If the PCV valve on a gasser gets stuck in the closed position, the gases will flow out the breather (crankcase inlet) instead and guck-up the outside of the engine or the inside of the air filter box, depending where the crankcase inlet originates.

 

[TDI Believer]

PCV valves are designed to be fully open at idle (high vacuum) and partially open at higher engine speeds (lower vacuum). If the PCV valve is stuck in the full open position, this will upset the air/fuel ratio at engine speeds above idle, causing the engine to run poorly because it is running too lean. If the PCV valve is stuck midway or fully closed, the engine will idle poorly or stall because it is running too rich. On newer computer controlled, fuel injected gassers the effect may not be as noticeable since the computer can adjust for (and therefore mask) such malfunctions.

On a gasser, crankcase gases must be metered to the engine to keep the air/fuel mixture at the correct ratio. This is not necessary on a diesel since there is no air/fuel mixing in the intake system. Diesels simply draw in air, and the more the better.

So in correction to what I said earlier, the PCV valve is more than just a check valve. It is a metering valve as well. The stopper in the PCV valve is weighted precisely to be open the correct amount at various engine speeds and levels of vacuum.

[This message has been edited by TDI Believer (edited June 01, 2000).]

 

[mojogoes]

Would this change depending on how much boost pressure you were running.......i.e positive pressure / blow-by the more boost you run.

 

[Bob_Fout]

Mojo the last post was 8.5 years ago lol.